Disease relevance of srtA

• A Staphylococcus aureus mutant defective in the anchoring of surface proteins was isolated and shown to carry a mutation in the srtA gene [1].
• Overexpression of srtA increased the rate of surface protein anchoring, and homologs of srtA were found in other pathogenic Gram-positive bacteria [1].
• Inactivation of the srtA gene in Listeria monocytogenes inhibits anchoring of surface proteins and affects virulence [2].
• METHODS: Wild-type and corresponding mutants with deletions of the srtA and/or srtB genes were used in murine acute lethal infection, septic arthritis, kidney infection and rat endocarditis models [3].
• Together these results indicate that sortase B (srtB) plays a contributing role during the pathogenesis of staphylococcal infections, whereas sortase A (srtA) is an essential virulence factor for the establishment of septic arthritis [4].

High impact information on srtA

• Staphylococcus aureus sortase, an enzyme that anchors surface proteins to the cell wall [1].
• The NMR structure of sortase reveals a unique beta-barrel structure, in which the active-site sulfhydryl of cysteine-184 is poised for ionization by histidine-120, presumably enabling the resultant thiolate to attack the LPXTG peptide [5].
• Replacement of the single conserved cysteine at position 184 of sortase with alanine abolishes enzyme activity [6].
• Staphylococcus aureus sortase A cleaves surface protein precursors bearing C-terminal LPXTG motif sorting signals between the threonine and glycine residues [7].
• Using lipid II precursor as cosubstrate, sortase A catalyzes the amide linkage between the carboxyl group of threonine and the amino group of pentaglycine cross-bridges, thereby tethering C-terminal ends of surface proteins to the bacterial cell wall envelope [7].

Chemical compound and disease context of srtA

• Surface proteins of Staphylococcus aureus are linked to the bacterial cell wall by sortase, an enzyme that cleaves polypeptides at the threonine of the LPXTG motif [6].
• It has been suggested that the sortase from Staphylococcus aureus (SrtA), may use a similar catalytic strategy as the papain cysteine proteases, holding its Cys184 side chain in an active configuration through a thiolate-imidazolium ion interaction with residue His120 [8].
• Anchoring of surface proteins to the cell wall of Staphylococcus aureus. Cysteine 184 and histidine 120 of sortase form a thiolate-imidazolium ion pair for catalysis [9].
• Anchoring of surface proteins to the cell wall of Staphylococcus aureus. A conserved arginine residue is required for efficient catalysis of sortase A [10].
• Sortase, a membrane protein of Staphylococcus aureus, cleaves polypeptides between the threonine and the glycine of the LPXTG motif and catalyses the formation of an amide bond between the carboxyl-group of threonine and the amino-group of peptidoglycan cross-bridges [11].

Biological context of srtA

• Two genes encoding putative sortases, termed srtA and srtB, were identified in the genome of the intracellular pathogenic bacterium Listeria monocytogenes [2].
• This phenotype was complemented when a wild-type srtA gene was provided in trans [12].
• Effect of srtA and srtB gene expression on the virulence of Staphylococcus aureus in animal models of infection [3].
• Comparison of the active sites of S. aureus sortase A and sortase B provides insight into substrate specificity and suggests a universal sortase-catalyzed mechanism of bacterial surface protein anchoring in Gram-positive bacteria [13].
• Hydrolysis and transpeptidation required the sulfhydryl of cysteine 184, suggesting that sortase catalyzed the transpeptidation reaction of surface protein anchoring via the formation of a thioester acyl-enzyme intermediate [14].

Anatomical context of srtA

• Thus, L. monocytogenes srtA is required for the cell wall anchoring of InlA and, presumably, for the anchoring of other LPXTG-containing proteins that are involved in listerial infections [2].
• Furthermore, the srtA mutant had reduced binding to immobilized human fibronectin and had a decreased ability to colonize the oral mucosa of mice [15].
• This phenomenon demonstrates the involvement of host neutrophils; when these cells were depleted, sortase mutant staphylococci caused severe systemic infection, although not septic arthritis [16].

Associations of srtA with chemical compounds

• S. aureus strain Newman variants lacking the capacity for synthesizing polysaccharide capsule (capFO), poly-N-acetylglucosamine (icaAC), lipoprotein (lgt), cell wall-anchored proteins (srtA), or the glycolipid anchor of lipoteichoic acid (ypfP) bound GFP-CWT similar to wild-type staphylococci [17].
• Sortase cleaves surface proteins at the LPXTG motif and catalyzes the formation of an amide bond between the carboxyl group of threonine (T) and the amino group of cell-wall crossbridges [5].
• Sortase cleaves polypeptides between the threonine and the glycine of the LPXTG motif [9].
• The three-dimensional structure of sortase revealed the close proximity of the catalytic site residue cysteine 184 with histidine 120; however, no structural evidence for a thiolate-imidazolium ion pair could be detected [9].
• Further, alanine substitution of tryptophan 194, a residue that is in close proximity of histidine 120, reduces the transpeptidase activity of sortase [9].

Other interactions of srtA

• In addition to the family of adhesive proteins bound to the cell wall via the sortase A (srtA) mechanism, secreted proteins such as the fibrinogen-binding protein Efb, the extracellular adhesion protein Eap, or coagulase have been found to interact with various extracellular host molecules [18].
• Sortase acts on surface proteins that are initiated into the secretion (Sec) pathway and have their signal peptide removed by signal peptidase [11].
• Recently, it has been shown in Staphylococcus aureus that the covalent anchoring of protein A, a typical LPXTG protein, is due to a cysteine protease, named sortase, required for bacterial virulence [19].
• In contrast, mutants deficient in sortase A or clumping factor B (ClfB) showed reduced nasal colonization [20].
• Inhibition of sortase-mediated Staphylococcus aureus adhesion to fibronectin via fibronectin-binding protein by sortase inhibitors [21].

Analytical, diagnostic and therapeutic context of srtA

• OBJECTIVE: The role that the surface proteins anchored by the srtA and srtB gene products play in the ability of Staphylococcus aureus bacteria to establish infection was investigated in several animal models [3].
• CONCLUSION: Results from this study substantiate the role of the srtA gene product in the establishment of infections and further studies are warranted to define and exploit this as a target for antimicrobial chemotherapy [3].
• Mean bacterial counts in cardiac vegetations were significantly higher for the wild-type and srtB- strain compared with the srtA- and srtA-B- strains [3].
• Additionally, using Western-blot analysis, we have been able to demonstrate that a biotinylated version of the peptidyl-diazomethane analogue, biotin-Ahx (aminohexanoyl)-Leu-Pro-Ala-Thr-CHN(2) (III), can be used as an affinity label to detect the presence of wild-type SrtA in crude cell lysates prepared from S. aureus [22].
• In addition, using immunofluorescence, cell adhesion assays, and transmission electron microscopy, we establish links between in vitro substrate specificity and in vivo function of the S. aureus sortase isoforms [23].

References